Following 2 days of fever, progressive confusion, and abdominal pain, a 55-year-old man is… — USMLE Step 2 CK

Following $2$ days of fever, progressive confusion, and abdominal pain, a $55$ -year-old man is brought to the emergency department. He has a known history of alcohol-associated cirrhosis and diuretic-refractory ascites, for which he takes furosemide and spironolactone. His temperature is $38.6^\circ\text{C}$ ( $101.5^\circ\text{F}$ ), blood pressure is $88/54$ mm Hg, pulse is $112$ /min, and respirations are $24$ /min. Physical examination reveals scleral icterus, a distended abdomen with shifting dullness, and diffuse abdominal tenderness to palpation. Asterixis is present.

Laboratory studies show:
Sodium: $140$ mEq/L
Potassium: $2.9$ mEq/L
Chloride: $105$ mEq/L
Bicarbonate: $20$ mEq/L
Blood urea nitrogen: $42$ mg/dL
Creatinine: $1.9$ mg/dL
Albumin: $1.0$ g/dL
Lactate: $5.2$ mmol/L

Arterial blood gas on room air:
pH: $7.51$
PaCO $_2$ : $26$ mm Hg
PaO $_2$ : $85$ mm Hg

Which of the following best characterizes this patient's primary acid-base disturbances?

A
Normal anion gap metabolic acidosis and respiratory alkalosis
B
High anion gap metabolic acidosis and respiratory alkalosis
High anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosisAnswer
D
Normal anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis
E
High anion gap metabolic acidosis, metabolic alkalosis, and respiratory acidosis

Answer

The correct answer is high anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis.

The correct answer is high anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis. This patient's calculated anion gap is 15 mEq/L, which might appear normal or borderline depending on the laboratory reference range. However, adjusting for his severe hypoalbuminemia (1.0 g/dL) yields a corrected anion gap of 22.5 mEq/L, confirming a high anion gap metabolic acidosis (lactic acidosis due to sepsis from spontaneous bacterial peritonitis). Calculating the delta gap reveals an excess anion gap of 10.5 mEq/L. Subtracting this from a normal bicarbonate of 24 mEq/L gives an expected bicarbonate of 13.5 mEq/L. Since his actual bicarbonate (20 mEq/L) is higher than expected, a concurrent metabolic alkalosis is present (diuretic-induced contraction alkalosis). Finally, Winter's formula predicts an expected PaCO2 of 38 mm Hg. The actual PaCO2 is 26 mm Hg, indicating a concurrent primary respiratory alkalosis.

Step-by-Step Solution

Calculate the uncorrected anion gap (AG) using the formula

\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)

140 - (105 + 20) = 15

mEq/L.

To screen for an unmeasured anion accumulation (normal range is approximately

8-12

mEq/L).

Correct the calculated anion gap for the patient's severe hypoalbuminemia.

Add

2.5

mEq/L to the AG for every

1.0

g/dL the albumin is below

4.0

g/dL. Corrected AG

= 15 + [2.5 \times (4.0 - 1.0)] = 15 + 7.5 = 22.5

mEq/L.

Albumin is a major unmeasured anion; severe hypoalbuminemia falsely lowers the calculated AG, masking a primary high AG metabolic acidosis. The corrected AG of

22.5

mEq/L confirms a high anion gap metabolic acidosis (due to lactic acidosis from sepsis).

Calculate the delta-delta gap (

\Delta\text{AG} / \Delta\text{HCO}_3^-

) to check for a concurrent metabolic disorder.

Excess AG

= 22.5 - 12 = 10.5

mEq/L. Expected

\text{HCO}_3^- = 24 - 10.5 = 13.5

mEq/L. The patient's actual

\text{HCO}_3^-

20

mEq/L.

Since the actual

\text{HCO}_3^-

(

20

mEq/L) is significantly higher than the expected

\text{HCO}_3^-

(

13.5

mEq/L), there is a secondary process adding bicarbonate to the serum. This confirms a concurrent metabolic alkalosis (likely contraction alkalosis from diuretic use).

Assess the respiratory compensation using Winter's formula.

Expected PaCO

_2

= (1.5 \times 20) + 8 = 38 \pm 2

mm Hg. The actual PaCO

_2

26

mm Hg.

Since the actual PaCO

_2

(

26

mm Hg) is much lower than the expected PaCO

_2

(

36-40

mm Hg), a primary respiratory alkalosis is also present (driven by hyperventilation from sepsis and hepatic encephalopathy).

Key Concept

Identifying a triple acid-base disorder requires stepwise calculation: correcting the anion gap for albumin, evaluating the delta gap, and applying Winter's formula.

Estimated Time:3m 0s

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